Biodiesel is a renewable, clean-burning petroleum diesel replacement that enhances independence from imported petroleum, reduces greenhouse gas emissions, supports agriculture and rural economies, and creates jobs.
While biodiesel provides many benefits, biodiesel production must be economical in order to maintain supply of the advanced biofuel. Producers must adapt to changing market conditions with new processes for converting low-cost feedstocks while meeting stringent product quality specifications.
Biodiesel finished product quality standards have evolved over recent years. Currently, to ensure product consistency and protect the consumer, biodiesel quality is regulated according to various commercial standards, including ASTM D6751, EN 14214, CAN/CGSB 3.524, and numerous customer-specific specifications. The aforementioned specifications require biodiesel to be produced with tight tolerances for many properties, including flash point, residual alcohol, water and sediment, kinematic viscosity, sulfated ash, oxidation stability, sulfur, copper strip corrosion, cetane number, cloud point, carbon residue, Acid Number, cold soak filterability, monoglycerides, total and free glycerin, phosphorous, 90% distillation temperature, calcium and magnesium, sodium and potassium, particulate contamination, and ester content. The most recent revision of ASTM D6751, D6751-12, introduced multiple biodiesel grades with different limits for Cold Soak Filtration test time and monoglyceride content, further increasing the importance of these two properties for customer acceptance of biodiesel.
As specifications for biodiesel become more rigorous than anticipated by earlier designers of production processes and as demand for lower cost and non-food feedstocks increases, biodiesel producers have an urgent need to improve their production processes to allow the use of new and/or low-cost feedstocks in order to compete and remain economically viable. However, low cost feedstocks contain a variety of low level impurities that can negatively impact biodiesel quality according to the aforementioned commercial specifications.
Corn oil is an example of a promising lower cost, non-food biodiesel feedstock that contains impurities that prevent traditional biodiesel plants from using it to produce biodiesel that meets all commercial specifications. In 2005, the U.S. produced 42 percent of the world's corn. As of September 2012, the United States had the nameplate capacity to produce approximately 14 billion gallons of ethanol in 211 operational ethanol plants. As of only a few years ago, market conditions changed allowing it to be profitable for ethanol producers to separate corn oil from the byproducts of ethanol production. To demonstrate the potential volume of corn oil that could be recovered from ethanol plants, a 100 MGPY ethanol plant is theoretically able to produce 7 million gallons of corn oil annually.
Much of the corn oil that has been recovered to date has been sold for use in animal feeds and for industrial uses since it poses a number of challenges for biodiesel production. Corn oil contains wax compounds that cause biodiesel to fail the Cold Soak Filtration test in ASTM D6751 when processed in traditional biodiesel production processes. It has proven difficult to remove these waxes from the finished biodiesel product, in part because of their solubility in biodiesel alkyl esters across a wide temperature range. Waxes can be partially removed using cold filtration technology or other winterization techniques. An embodiment of the invention disclosed herein successfully removes a vast majority of these waxes so as to efficiently meet ASTM D6751 specification requirements.
Corn oil from ethanol plants also contains elevated free fatty acid (FFA) content. The FFA content of this corn oil may be between about 4 and 15 wt %. In general, high FFA feedstocks are difficult to process into biodiesel by base-catalyzed transesterification because the FFAs are converted to soaps leading to undesirable processing consequences (e.g., emulsion formation and increased catalyst costs), yield losses, and production rate downturns. The invention disclosed herein allows nearly any feedstock to be processed, regardless of its initial FFA content. As described below in detail, the invention includes multiple embodiments for reducing FFA in the feedstocks (i.e., deacidifying them) prior to transesterification, including conversion to soaps followed by physical removal, physical removal by distillation, and/or chemical conversion by esterification with an alcohol, such as methanol, ethanol, or glycerol.
In some cases even after feedstock pretreatment with an FFA reduction process, residual FFA levels can still remain higher than desirable for traditional biodiesel production, which can result in higher Acid Number values in the finished biodiesel. This is of particular concern for biodiesel produced from feedstocks such as corn oil and some fatty acid distillates. When corn oil, corn oil biodiesel, and fatty acid distillates (along with biodiesel produced therefrom) are analyzed for Acid Number with ASTM D664 Method B, they reveal a second inflection point in the titration curve caused by compounds that are neutralized after the free fatty acids. This additional inflection point causes the feedstock to exhibit an Acid Number greater than would be predicted by its true FFA content, and this Acid Number increase can be imparted to the resulting biodiesel. One of the embodiments of the invention disclosed herein efficiently reduces the quantities of both FFAs and the compounds that cause the second inflection point such that the finished biodiesel more easily and more predictably meets commercial biodiesel specifications for Acid Number.
In addition to having higher FFA levels than conventional commodity fats and oils, lower cost, non-food feedstocks for biodiesel are often much darker in color and higher in sulfur content. In traditional production processes, the darker color and a significant portion of the sulfur content are largely imparted to the finished biodiesel, which can create barriers to meeting commercial specifications and to customer acceptance in general. For example, corn oil sourced from ethanol processes customarily retains a deep red color. The red color of the resulting biodiesel gives the appearance of fuel that has been dyed, which is the established governmental regulatory method to clearly advise wholesalers, retailers and consumers that a diesel product is for off-road use only. Red-dyed diesel fuel has critical tax implications and is therefore strictly regulated. In commercial distribution of biodiesel made from ethanol-sourced corn oil, fuel retailers and end-users have expressed deep concern about using this fuel for on-road applications, even to the point of refusing to accept the product. It is important to overcome this failure of market acceptance. Our research indicates that this red coloration can be reduced to acceptable orange, yellow, or even clear colorations depending on the embodiment of this invention that is chosen for processing the feedstock and purifying the biodiesel. For example, some biodiesel filter aids may reduce the intensity of the red color, but only to a limited extent. It is costly and inefficient to sufficiently eliminate the red color by use of biodiesel filter aids alone. However, removing free fatty acids from the corn oil by distillation and then purifying the eventual biodiesel with filter aids will produce biodiesel with a commercially acceptable color.
Similarly, certain filter aids can reduce the sulfur content of biodiesel made from lower cost, non-food feedstocks with high sulfur levels, but again this process is cost prohibitive to produce biodiesel that is competitively priced with petroleum diesel. An embodiment of the present invention efficiently removes the impurities that cause unacceptable colors and/or high sulfur content of the finished biodiesel product, thereby providing a fuel which will have unrestricted acceptance in the market.
In addition to higher sulfur content and coloration issues, lower cost, non-food feedstocks can also contain significant quantities of high molecular weight, low volatility unsaponifiable components which are soluble in both the oil and the resulting biodiesel and therefore cannot be easily removed in conventional biodiesel processes. The presence of these impurities may lower the perceived quality of the finished biodiesel product and/or impact its performance in certain operating conditions. Further, such impurities reduce the ester content of the finished biodiesel and thereby create potential specification issues under EN 14214, CAN/CGSB 3.524, and numerous customer-specific specifications in the United States. Corn oil in particular contains markedly high levels of unsaponifiable components. An embodiment of the invention disclosed efficiently removes these unsaponifiable impurities to produce a higher quality biodiesel with improved market acceptance.
Although the supply of corn oil is expected to increase significantly in the near future, the characteristics described above pose significant challenges for biodiesel producers who wish to make and market biodiesel made from it. Similar biodiesel quality and customer acceptance obstacles also impede the current and future use of other emerging low cost, non-food feedstocks for biodiesel production, including used cooking oils, poultry fats, brown grease, fatty acid distillates, pennycress oil, and algal oils.
In sum, the biodiesel industry has historically used a majority proportion of higher purity feedstocks (often edible oils and fats) and has been restricted by fewer and less stringent product acceptance specifications. As the industry and its customers have evolved, pricing and availability of higher purity feedstocks have pushed the industry to explore the use of lower cost, less pure feedstocks while it simultaneously faces tightening acceptance specifications and commercial requirements for the finished product. Further, these lower cost feedstocks are not consistent in the nature and content of their impurities and exhibit great variation based not only on the underlying source of the oil, but on its production process and other variables associated with the recovery of the oils from their source materials. Conventional oil degumming pretreatment processes alone will no longer allow production of biodiesel that is universally commercially acceptable. As a result, what has been absent in the biodiesel production processes are methods, systems, and compositions that allow biodiesel producers to economically convert lower cost, non-food feedstocks such as corn oil, used cooking oil, poultry fats, brown grease, fatty acid distillates, pennycress oil, and algal oils into high-quality biodiesel that can and will conform to the various commercial biodiesel specifications in their current and future forms. More specifically, it is necessary to be able to produce biodiesel from such feedstocks via methods and systems that overcome potential specification problems associated with feedstocks that contain any combination of a variety of problem properties such as waxes, unsaponifiables, varying FFA levels, unacceptable color, and high sulfur levels.